Japanese Patent Publication No. 2004-521235 discloses a lift-variable valve-operating mechanism in which a cam shaft 3 and a gas exchange valve (engine valve) 4 are connected through a rotating lever (subsidiary cam) 21 and a cam lever (rocker arm) 5, and a relationship of a lift amount of the gas exchange valve 4 with respect to a phase of the cam shaft 3 is made variable by changing a position of a rocking fulcrum of the rotating lever 21 by a controller 25. In this mechanism, one end of the cam lever 5 abuts a stem end of the gas exchange valve 4 and the other end thereof is urged by a liquid-pressure-type valve-clearance compensating element (hydraulic tappet) 7, thereby adjusting a clearance of a driving-force transmission path extending from the cam shaft 3 to the gas exchange valve 4.
In this type of lift-variable valve-operating mechanism, it is important to appropriately adjust the lift amount of the engine valve, but the adjustment can be made only while an internal combustion engine is stopped. However, if the internal combustion engine is stopped, a hydraulic pump driven by a crankshaft is also stopped, bringing the hydraulic tappet into an inoperative state. As a result, the rocking fulcrum of the rocker arm becomes unstable, leading to a problem that adjustment of the lift amount becomes impossible.
In order to solve this problem, in the mechanism described in Japanese Patent Publication No. 2004-521235, an outer peripheral groove 15 is formed at the tip end of a piston 6 in a liquid-pressure-type valve-clearance compensating element (hydraulic tappet) 7; a fork-shaped end portion 9′ of a tool 9 is engaged with the outer peripheral groove 15; and the fork-shaped end portion 9′ is pulled by a tension spring 16. With this arrangement, even in a state where liquid pressure is not supplied to the liquid-pressure-type valve-clearance compensating element 7, it is possible to adjust the clearance of the driving-force transmission path extending from the cam shaft 3 to the gas exchange valve 4, thereby adjusting the lift amount of the engine valve.
However, the mechanism described in Japanese Patent Publication No. 2004-521235 requires the outer peripheral groove 15 to be formed at the tip end of the piston 6 of the liquid-pressure-type valve-clearance compensating element 7 so that the outer peripheral groove 15 can be engaged with the fork-shaped end portion 9′ of the tool 9. Thus, in order to compensate for a lowered strength of the piston 6 due to formation of the outer peripheral groove 15, it is disadvantageously required to increase the size or wall thickness of the piston, and also there is a concern that the piston 6 is twisted by the tool 9 to lower durability of the liquid-pressure-type valve-clearance compensating element 7.